California Institute for Energy and Environment (CIEE)

This research is part of a spectrum of studies of the California water system to assess impacts of climate change on urban and agricultural water agencies. This report describes preliminary work on methods for measuring current water supply reliability and methods for projecting changes in supply reliability caused by climate change, including: (1) a review of recent climate change literature in California; (2) a summary of criteria for evaluating water resource models; (3) an assessment of CALSIM-II water supply reliability forecasts and (4) an assessment of the accuracy of April-June flow forecasts performed by the California Department of Water Resources (DWR). The literature review showed that climate change will affect Californian hydrology in several ways, including an earlier start of spring snowmelt, an increase in winter runoff as a fraction of total runoff, and an increase in winter flood frequency. The study’s evaluation of three models used to estimate the water supply impacts of changing climate scenarios—CALSIM-II, CALVIN, and CVMod—revealed each model strengths and weaknesses. The CALSIM-II studies revealed differences in water supply reliability among user groups and showed that supply reliability at the aggregate basin level differs markedly from reliability at the levels of Depletion Study Area (DSA) and other sub groups within basins. Additionally, the distribution of reliability is different between annual and monthly terms. Finally, analysis of DWR data showed a positive correlation between forecast accuracy and watershed elevation, and showed wet year forecasts to be significantly more accurate than dry year forecasts. This finding implies a strong correlation between snowpack and ability to predict streamflow, and thus a reduction in forecast reliability under most climate change scenarios.